Advertising in online mediums have some similarities with traditional print-based advertising. For example, it is usually desirable to target the advertisement to the intended or expected audience, because advertisers generally prefer to invest in advertising if it reaches many potential customers, as opposed to persons who have little or no interest in the product or service being advertised. In print media, this targeting could be based on the general knowledge about who the readers are, or market investigations can be performed on the readers to more accurately determine their interests.
In the online environment, publishers sometimes try to match the advertisement displayed on a page to the particular page as well as possible. Sometimes the matching involves determining one or more terms relating to the page and using this term to select one or more advertisements to be included in the page.